Polymethylmethacrylate (PMMA) bone cements are based on the pioneering work of Sir Charnley. PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component, which is also referred to as bone cement powder, comprises one or more polymers, a radiopaquer, and the initiator dibenzoylperoxide. The polymers of the powder component are produced on the basis of methylmethacrylate and comonomers, such as styrene, methylacrylate or similar monomers by means of polymerisation, preferably by suspension polymerisation. During the mixing of powder component and monomer component, swelling of the polymers of the powder component in the methylmethacrylate generates a dough that can be shaped plastically and is the actual bone cement. During the mixing of powder component and monomer component, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide while forming radicals. The radicals thus formed trigger the radical polymerisation of the methylmethacrylate. Upon advancing polymerisation of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Methylmethacrylate is the monomer used most commonly in polymethylmethacrylate bone cements. Redox initiator systems usually consist of peroxides, accelerators and, if applicable, suitable reducing agents. Radicals are formed only if all ingredients of the redox initiator systems act in concert. For this reason, the ingredients of the redox initiator system in the separate starting components are arranged appropriately such that these cannot trigger a radical polymerisation. The starting components are stable during storage provided their composition is adequate. Only when the two starting components are mixed to produce a cement dough, the ingredients of the redox initiator system, previously stored separately in the two pastes, liquids or powders react with each other forming radicals which trigger the radical polymerisation of the at least one monomer. The radical polymerisation then leads to the formation of polymers while consuming the monomer, whereby the cement dough is cured.
PMMA bone cements can be mixed by mixing the cement powder and the monomer liquid in suitable mixing beakers with the aid of spatulas. One disadvantage of said procedure is that air inclusions may be present in the cement dough thus formed and can cause destabilisation of the bone cement later on. For this reason, it is preferred to mix bone cement powder and monomer liquid in vacuum mixing systems, since mixing in a vacuum removes air inclusions from the cement dough to a large extent and thus achieves optimal cement quality. Bone cements mixed in a vacuum have clearly reduced porosity and thus show improved mechanical properties. A large number of vacuum cementing systems have been disclosed of which the following shall be listed for exemplary purposes: U.S. Pat. Nos. 6,033,105 A, 5,624,184 A, 4,671,263 A, 4,973,168 A, 5,100,241 A, WO 99/67015 A1, EP 1 020 167 A2, U.S. Pat. No. 5,586,821 A, EP 1 016 452 A2, DE 36 40 279 A1, WO 94/26403 A1, EP 1 005 901 A2, U.S. Pat. No. 5,344,232 A. An external vacuum pump is connected In the vacuum cementing systems thus specified to generate the negative pressure.
Cementing systems, in which both the cement powder and the monomer liquid are already packed in separate compartments of the mixing systems and are mixed with each other in the cementing system only right before application of the cement, are a development of cementing technology. Said full-prepacked mixing systems were proposed through EP 0 692 229 A1, DE 10 2009 031 178 B3, U.S. Pat. Nos. 5,997,544 A, 6,709,149 B1, DE 698 12 726 T2, and U.S. Pat. No. 5,588,745 A. Patent DE 10 2009 031 178 B3 discloses a closed vacuum mixing system having a two-part dispensing plunger for closure of a cement cartridge. A combination of a gas-permeable sterilisation plunger and a gas-impermeable sealing plunger is used in this context. This principle of a closed vacuum mixing system is implemented in the closed cementing system, PALACOS® PRO, made and distributed by Heraeus Medical GmbH.
Accordingly, it is the object of the invention to overcome the disadvantages of the prior art. In particular, a closed vacuum mixing system of a simple design and inexpensive to manufacture for polymethylmethacrylate bone cement and a method for the mixing of the two component bone cement in a vacuum are to be provided. An alternative solution shall be provided as well this context. Preferably, the vacuum mixing system is to have as few mobile parts as possible. In particular, the dispensing plunger is to have a simple design as well. The vacuum mixing system shall be easy and intuitive to operate.
Said vacuum mixing system is to contain a cement cartridge, in which the cement powder is stored, as well as a separate reservoir container in which the monomer liquid is present. Accordingly, the monomer liquid is stored separate from the cement powder. Any contact of the medical users with said components shall be excluded before and after the mixing of the two cement components, i.e. the cement powder and the monomer liquid. Therefore, the reservoir container must be opened and the monomer must be transferred in a closed system. The cement powder must not contact the medical user either. During an ethylene oxide sterilization of the vacuum mixing system, it is inevitable that the ethylene oxide penetrates into and then exits from the cement powder present in the cement cartridge. The cartridge must have an opening to the ambient atmosphere with a large gas exchange surface for said gas exchange. However, this cement powder must not exit from the cement cartridge during this process. This means that the vacuum mixing system must be designed appropriately such that a gas exchange is feasible with no cement powder exiting. Moreover, it must be possible to close the cement cartridge during the mixing process in vacuum-tight manner. It is therefore the object of the invention to provide a vacuum mixing device which reconciles the contradiction that exists between the maximal gas permeability of the cement cartridge for gas exchange during the ethylene oxide sterilization and concurrent prevention of the exit of cement powder as well as the vacuum tightness of the cement cartridge during the mixing process. The vacuum mixing system to be developed shall be made, if possible, from common thermoplastic materials by means of injection moulding and thus shall be suitable for single-use applications.
Moreover, a device that is inexpensive to manufacture and working reliably for the mixing of a medical cement and, if applicable, for storage of the starting components, and a method for the mixing of the bone cement is to be devised, in which a simple manual operation can be used to mix the starting components, if possible without air inclusions arising in the cement dough.
The main component of the polymethylmethacrylate bone cement shall be a powder and the second component shall be present in the form of a liquid. Preferably, it shall be possible to store the two starting components of the bone cement separate from each other in the vacuum mixing system and to combine them safely through the use of the device.